CN1972040A - Method of cutting stainless steel by using fiber laser - Google Patents

Abstract

A laser cutting method for cutting a stainless steel workpiece using a laser beam generating device comprising a quartz fiber having a core doped with ytterbium for generating a laser beam. Preferably, the wavelength of the laser beam generated by the ytterbium-based fiber is between 1.07 and 1.09 microns, the quality factor of the laser beam is between 0.33 and 8 mm.mrad, and the power of the laser beam is between 0.1 and 25 kilowatts. The auxiliary gas used for the laser beam is selected from nitrogen, helium, argon and mixtures thereof, in addition, it also optionally contains one or more additives selected from O ₂ , CO ₂ , H ₂ and CH ₄ compound.

Description

Method of Cutting Stainless Steel Using Fiber Laser

technical field

The invention relates to a laser cutting method for cutting stainless steel by using a fiber laser source doped with ytterbium.

Background technique

At present, CO2 _- type laser sources are widely used in the industrial field to generate laser beams for laser cutting, with a wavelength of 10.6 microns and a power of 6 kilowatts. This method is especially useful for cutting stainless steel.

However, the achievable cutting speed and the obtained cutting quality are very variable, depending on the material being cut and on the parameters of the cutting method used, such as the nature of the assist gas, the diameter of the focused beam, the power of the incident laser ,etc.

Therefore, _CO2 lasers cannot be used with assist gases of low ionization potential such as argon without the risk of creating parasitic plasmas that would be detrimental to the method.

Moreover, _CO2 lasers are limited in power, which directly affects cutting speed.

In addition, the laser beam must be guided exactly from the laser generator to the focusing head, ie the cutting head, which introduces imperfections, especially when aligning the optics in the optical path. This is because the optics used for guidance are usually polished and/or copper-coated mirrors whose position determines the path the laser beam travels. Therefore, these mirrors must be aligned precisely in order to ensure optimal entry of the laser beam into the focusing or cutting head. Today, the position of these mirrors is usually adjusted by mechanical means, but misalignment can easily occur due to component wear and changes in environmental conditions, especially ambient temperature, humidity, etc.

Furthermore, the optical path of the beam must be kept in an inert atmosphere to avoid any contamination and to maintain a medium with constant optical index, which is necessary for the beam to propagate well. These conditions are such that the properties related to the beam diameter and the lateral distribution of the beam energy as well as the beam quality properties can maintain the method to obtain satisfactory results, the figure of merit of the beam parameter product (BPP) of the high power _CO2 laser beam used in cutting Usually between 3mm.mrad and 6mm.mrad. This atmosphere also protects the optics used for guidance and prevents damage to these optics.

Now, this is impractical in industrial conditions and entails additional costs.

In order to be able to alleviate these problems, it has been proposed to use a Nd:YAG type laser device to cut stainless steel. A beam of light is generated and sent to a focusing head via an optical fiber.

However, this technical solution is also unsatisfactory from an industrial point of view.

This is because it has been found that cutting with a laser beam having a wavelength of 1.06 microns output from a Nd:YAG type laser source is inferior in terms of cutting quality and cutting speed.

This is because the quality factor (BPP value) of the Nd:YAG laser is not suitable for the laser cutting process, and its range is approximately between 15mm.mrad and 30mm.mrad depending on the laser source. Now, the higher the quality factor of a laser, that is, the larger the product between the focused beam waist and the beam divergence, the less efficient that laser beam is for the laser cutting process.

In addition, the lateral energy distribution of the focused Nd:YAG laser beam is not Gaussian, but has a top-hat distribution, and the lateral energy distribution becomes random after exceeding the focal point.

More generally, laser cutting with Nd:YAG lasers for cutting stainless steel is highly undesirable when an industrially acceptable cut speed and cut quality is desired.

Therefore, the problem arises how to provide an improved, industrially suitable method of cutting stainless steel with a laser beam, which can achieve speeds of up to 15 to 20 m/min or even higher depending on the thickness in question and excellent Excellent cutting quality, that is, straight cutting surface, no burr, and reduced roughness.

Contents of the invention

Therefore, the technical solution provided by the present invention is a laser cutting method for cutting a stainless steel workpiece, wherein a laser beam generating device comprising at least one ytterbium-containing fiber is used to generate a laser beam for melting the workpiece to perform the actual cutting The operation is characterized in that the quality factor of the laser beam is between 0.33 and 8mm.mrad.

The laser beam generating device comprises an exciter, preferably several exciters, which generate a laser beam in combination with at least one excited element, which is also called an amplification medium. The exciter is preferably several laser diodes, and the excited element is a fiber, preferably a quartz fiber with a core doped with ytterbium.

For the purposes of the present invention, no distinction will be made between the terms "laser beam generating device" and "resonator".

Depending on the circumstances, the method of the invention may comprise one or more of the following features:

- the fiber is formed of a core doped with ytterbium with a quartz cladding;

- said laser beam generated by said ytterbium-based fiber has a wavelength between 1 and 5 microns, preferably between 1.04 and 3 microns;

- the wavelength of the laser beam generated by said ytterbium-based fiber is between 1.07 and 1.09 microns, preferably 1.07 microns;

- the power of the laser beam is between 0.1 and 25 kW, preferably 0.5 to 15 kW;

- the laser beam is a continuous or pulsed laser beam, preferably a continuous laser beam;

- the thickness of the workpiece to be cut is between 0.25 and 30 mm, preferably between 0.40 and 20 mm;

- a cutting speed between 0.1 and 25 m/min, preferably between 2 and 20 m/min;

- the auxiliary gas used for the laser beam is selected from nitrogen, helium, argon and mixtures thereof, furthermore, the auxiliary gas may optionally contain one or more selected from _O2 , _CO2 , _H2 and _CH4 , etc. Various additional compounds;

- the quality factor of the laser beam is between 1 and 8 mm.mrad, preferably greater than 2 mm.mrad, more preferably greater than 3 mm.mrad, and/or preferably less than 7 mm.mrad, more preferably less than 5 mm.mrad;

- More generally, the assist gas pressure is between about 8 bar and 25 bar, which is chosen according to the thickness to be cut; and

- The diameter of the gas injection hole is between 0.5 and 4 mm, usually between 1 and 3 mm, which diameter increases with the thickness of the workpiece to be cut.

Description of drawings

Fig. 1 is for implementing the laser cutting method that utilizes laser beam to cut stainless steel workpiece;

Figure 2 shows the obtained speed as a function of the thickness to be cut;

Figure 3 shows the structure during the cutting of an incision in a material of thickness e;

Figure 4 shows the optimal angle α of the cutting front as a function of cutting thickness.

Detailed ways

1 is a schematic diagram of an installation for implementing a laser cutting method for cutting a stainless steel workpiece 10 with a laser beam 3 using a laser source 1 with a resonator or a laser beam generating device formed of a quartz fiber with a core doped with ytterbium 2 to generate a laser beam 3 .

A laser source 1 is used to generate a laser beam 3 having a wavelength of 1 micron to 5 microns, more precisely 1.07 microns.

The beam 3 propagates through a beam delivery device 4 , such as an optical fiber made of fused silica with a diameter of 20-300 μm, to the interaction zone 11 between the beam 3 and the workpiece 10 , ie the zone where the cut occurs.

On leaving the fiber 4, the laser beam 3 has specific optical properties and a figure of merit (BPP) between 1 and 8 mm.mrad.

The beam 3 is then collimated using an optical collimator 5 configured with a collimating doublet lens made of coated fused silica to limit the divergence of the beam leaving the fiber and to make the laser beam parallel.

Then, the parallel light beam 3 whose divergence has been largely limited by the collimator is focused onto or in the workpiece 10 to be cut through the coated fused silica lens 6, wherein the focal length of the fused silica lens 6 is 80 mm to 510 mm, preferably 100 mm to 250 mm.

Before reaching the workpiece 10, the beam 3 passes axially through a laser head 6 equipped with a nozzle 7 having an axial exit hole 8 facing the workpiece 10 to be cut, the beam 3 and the auxiliary Gas passes through the nozzle. The hole diameter of the nozzle may be between 0.5 mm and 5 mm, preferably between 1 mm and 3 mm.

The laser head 6 itself is fed into an auxiliary gas through the gas inlet 9, such as an inert gas, such as nitrogen, argon, helium or a mixture of some of these gases, it can also be some active gas, such as oxygen, or even a reactive gas. Gas/inert gas mixture.

The pressurized assist gas is used to remove molten metal from the cut 12 formed in the workpiece 10 as the workpiece 10 moves relative to the laser head 6 along the desired cutting path. Conversely, the same conclusion holds when the workpiece is held stationary while the cutting head is relatively moved.

Figure 3 shows the structure during cutting of the kerf (material of thickness e), showing the divergence angle θ of the laser beam after focusing, the diameter 2Wo of the focused beam and the angle α of the surface before cutting.

The beam quality factor or BPP is defined as the product of the divergence angle θ and its radius Wo.

The cutting process is controlled by the energy absorbed from the laser beam in the material during cutting. Depending on the wavelength of the laser beam employed, there is thus an optimum angle for the material to absorb energy. If the angle is not optimal, some energy will be reflected and/or lost.

Figure 3 shows that, under optimal cutting conditions, the angle α of the cutting front surface corresponds to exposing the entire thickness e of the material to a beam of diameter 2Wo.

Figure 4 shows the optimum angle α of the cut front surface as a function of cut thickness. The upper curve was obtained using a 4 kW TEM01 ^* mode _CO2 laser, while the lower curve was obtained using a 2 kW ytterbium-based fiber laser according to the present invention. The two curves do not coincide because of the difference in the angle of optimum energy absorption at 10.6 microns, which is the wavelength of the _CO2 laser, and 1.07 microns, which is the wavelength of the ytterbium-based fiber laser.

From these curves it is clearly evident that the optimum angle of the cut front surface is greater for smaller thicknesses than for larger thicknesses. The maximum angle at which the laser energy is transmitted into the material is obtained geometrically, which is the sum of two angles, ie α+θ.

Therefore, it is understandable that when cutting objects of small thickness (a few millimeters), a low beam divergence, that is, a small BPP, is required because the spot diameter is set by the fiber diameter used to maintain optimum The energy absorption angle is constant.

It can also be deduced from this that beyond a value of 8 mm.mrad the efficiency of energy transfer from the beam to the material decreases.

Therefore, for the present invention, the quality factor of the employed laser beam is preferably 1 to 8 mm.mrad, more preferably 2 to 8 mm.mrad.

example

In order to illustrate the effectiveness of the method of the invention, several cutting experiments were carried out on stainless steel workpieces using a resonator comprising an amplifying medium or a device for generating a laser beam according to the method of the invention for generating a laser beam The device consisted of a silica fiber with a core doped with ytterbium, and the results of these experiments are given in the Examples below.

More precisely, the laser source employed in the following examples includes an amplifying medium consisting of a diode-excited ytterbium-doped fiber, producing a laser beam with a power of 2 kilowatts and a wavelength of 1.07 micrometers at 100 Micron-coated fused silica fibers were propagated and the fibers had a figure of merit (BPP) of 4 mm.mrad. The collimator located at the exit of the fiber was configured with a doublet lens with a focal length of 55 mm.

In order to determine the range of speeds achievable with the method of the present invention depending on the thickness of the workpiece to be cut and the gas pressure and composition of the assist gas employed, cutting tests were performed on stainless steel workpieces with a thickness of 1.5 mm to 8 mm.

The gas employed is an inert gas, i.e. nitrogen, and is injected into the interaction zone where the beam interacts with the workpiece and through the laser cutting nozzle, at a gas pressure varying in the range of 8 to 25 bar depending on the gas employed, The diameter of the hole of the laser cutting nozzle varies in the range of 0.5 to 4 mm, usually 1 to 3 mm according to actual conditions. The thicker the object being cut, the larger the diameter of the nozzle must be.

A focusing lens with a focal length between 127 mm and 190.5 mm is used to focus the laser beam generated by an amplifying medium comprising diode-excited ytterbium-doped fibers and delivered to the cutting head by means of optical delivery The focusing lens, the optical delivery device is, for example, a 100 micron diameter optical fiber.

More precisely, a lens with a focal length of 127 mm is used for objects to be cut with a thickness of 4 mm or less, and a lens with a focal length of 190.5 mm for other greater thicknesses.

The results obtained using a nitrogen pressure of 15 bar, which is satisfactory from the point of view of the quality of the cut obtained, are given in Figure 2, which shows the speed obtained (points on the y-axis) as a function of the Variation in thickness (points on the x-axis).

The figure shows that, on a 2 mm thick slab, under the conditions described above, the method of the invention makes it possible to achieve a speed of 16 m/min. However, the figure also shows that, logically, the cutting speed decreases as the thickness of the cut material increases.

Furthermore, it should be emphasized that after inspection of the cut surfaces, the cuts obtained were very satisfactory in terms of burrs and streaks for all cut thicknesses.

However, the maximum thickness cut with the laser power used here was 8 mm under the above experimental conditions.

Therefore, the cutting speed and cutting quality of the method of the present invention are effective for stainless steel cutting.

Claims (10)

1. laser cutting method that is used for the cutting stainless steel workpiece, thereby wherein use laser-beam generating apparatus to melt described workpiece and carry out actual cutting, wherein said laser-beam generating apparatus comprises at least a ytterbium fiber that contains, be used to produce laser beam, it is characterized in that the quality factor of described laser beam are between 0.33 to 8mm.mrad.

2. method according to claim 1 is characterized in that, described fiber is formed by the core of the doping ytterbium with quartzy covering.

3. method according to claim 1 and 2 is characterized in that, the wavelength of the described laser beam that is produced by described ytterbium-based fiber is preferably between 1.04 to 3 microns between 1 to 5 micron.

4. according to the described method of one of claim 1 to 3, it is characterized in that the wavelength of the laser beam that is produced by described ytterbium-based fiber is preferably 1.07 microns between 1.07 to 1.09 microns.

5. according to the described method of one of claim 1 to 4, it is characterized in that the power of described laser beam is preferably between 0.5 to 15 kilowatt between 0.1 to 25 kilowatt.

6. according to the described method of one of claim 1 to 5, it is characterized in that described laser beam is continuous or pulse laser beam, is preferably continuous laser beam.

7. according to the described method of one of claim 1 to 6, it is characterized in that the thickness of the described workpiece that will cut is preferably between 0.40 to 20 millimeter between 0.25 to 30 millimeter.

8. according to the described method of one of claim 1 to 7, it is characterized in that described cutting speed is preferably between 2 to 20 meters/minute between 0.1 to 25 meter/minute.

9. according to the described method of one of claim 1 to 8, it is characterized in that the described assist gas that is used for laser beam is selected from nitrogen, helium, argon gas and composition thereof, in addition, described assist gas also comprises and is selected from O ₂, CO ₂, H ₂And CH ₄One or more additional compounds.

10. according to the described method of one of claim 1 to 9, it is characterized in that the quality factor of described laser beam are between 1 to 8mm.mrad, be preferably more than 2mm.mrad, more preferably greater than 3mm.mrad, and/or preferably less than 7mm.mrad, more preferably less than 5mm.mrad.